1. Field of the Invention
The present invention relates to an audio signal processing apparatus and, more particularly, to an apparatus adapted to process input audio signals digitally in accordance with the contents of a program for suitably approximating or creating a desired sound field.
2. Description of the Related Art
In such auditory places as concert halls, opera houses and cabaret-like establishments giving live concerts, there is created a sound field which, with its own beautiful resonant effects, is specific to each place. How to reproduce such esthetic resonance at the highest fidelity possible in the personal listening room has long been one of the major objectives pursued by audiophiles.
Recently there have been developed and commercialized sound field creation systems for reproducing in an individual's room various sound fields approximating those of concert halls and the like. These systems are designed to create a desired sound field by digitally processing audio signals. The digital processing of these signals is based on the data gained by analyzing the acoustic space configurations of diverse places where concerts and other live performances are held. What follows is a description of the concept for artificially creating desirable sound fields.
Assume that musical instruments are being played in a concert hall, and that the sound field inside (made up of sound waves with characteristic differences in terms of delay time, direction of incoming sound, decibel level, etc.) is being observed on the time base. It will be understood that the sound waves occur in two kinds: the sounds coming directly to the audience from the instruments (hereinafter called direct sounds), and the sounds reaching the human ears after reflection against the walls and the floor of the hall (hereinafter called reflected sounds). The reflected sounds may be further divided by nature into two groups: initially reflected sounds, and subsequently reverberated sounds.
Initially reflected sounds occur about 50 to 100 milliseconds after the direct sounds reach the audience. The frequency of the reflections involved is relatively low. The presence of initially reflected sounds, or the lack of them, affects the degree of articulation of the conversations in a play and lyrics of songs that the audience can hear. In terms of auditory perception, initially reflected sounds help the audience form their sense of spaciousness and depth inside the hall. Meanwhile, subsequently reverberated sounds reach the audience following the initially reflected sounds. Echoed repeatedly against the walls and the floor and absorbed by their material upon every reflection, subsequently reverberated sounds progressively drop their decibel levels and tend to dissipate in every direction. As they reach the audience, subsequently reverberated sounds are perceived as coming in all directions with equal degrees of probability. These sounds constitute the auditory effects of resonance, richness, and ambience specific to the hall. Too high levels of the reverberated sounds tend to reduce the articulation of the conversation in a play and lyrics of songs reaching the audience.
The data gained from analyzing the acoustic space of auditory places contain numerous parameters, including initial delay, room size, liveness and reverb time. By modifying these parameters, it is possible to adjust the constitution of the echoed sounds and the audience's subjective impression affected by such sounds. Below is a description of such adjustable parameters. It should be noted that setting the same parameters for different concert halls does not create the same sound field in such places. The obvious reason is that each concert hall has its own acoustic parameters.
The initial delay is a parameter that determines the positional relation between sound sources and the audience. This parameter varies the time that elapses from the moment a direct sound reaches the audience until the moment a first initially reflected sound does. The shorter the initial delay, the shorter the perceived distance to the sound sources from the audience. Conversely, larger initial delays increase the perceived audience-to-sound-source distance.
The room size is a parameter that determines the volume of a concert hall. As shown in FIG. 1, this parameter varies the time intervals (.DELTA.T.sub.1, .DELTA.T.sub.2) between initially reflected sounds. The smaller the set value of the parameter, the smaller the concert hall size perceived. Larger parameter settings make the concert hall perceived more extensive.
The liveness is a parameter that determines the degree of acoustics in a given auditory place. As depicted in FIG. 2, this parameter varies the gradient of the level of initially reflected sounds in relation to time. The smaller the set value of the parameter, the more "dead" the sound field becomes (i.e., initially reflected sounds attenuate rapidly). Larger parameter settings make the sound field more "live" (i.e., initially reflected sound attenuate slowly).
The reverb time is a parameter that determines the time over which subsequently reverberated sounds attenuate. The parameter sets the time required to let a sound, after its source has become inactive, attenuate by 60 dB. The smaller the set value of the parameter, the shorter the duration of reverberation. Larger parameter settings prolong the reverb time.
These parameters are stored in computer memory after being classified into such modes as HALL1 through HALL4, JAZZ1 through JAZZ4, ROOM1, ROOM2, ROCK1, ROCK2, DISCO1, DISCO2, and CHURCH, each representing a specific type of auditory place. When a desired mode is selected to create a sound field, the parameters corresponding to that mode have their set values executed to trigger digital processing of audio signals. The result is the substantially faithful reproduction of the sound field of the desired concert hall settings in the listener's own room.
The trouble with the setup above is that if the data in each of the parameters are fixed to the modes representing various concert halls, the sound field for each mode comes out always unchanged. To avoid this boring effect, the user can create his own data based on default data and store the unique data in the user's memory. Upon reproduction, a desired sound field is created based on the corresponding sound field.
When a sound field is to be created using user-specified parameter data, it is necessary to indicate on a display the information indentifying the data specifically created by the user. On such an occasion, it is desirable to have another indication helping the user readily know the mode of the sound field (i.e., concert hall) on which the desired sound field to be created is based.